In prior art systems where a plurality of perforating guns are connected in tandem to perforate a substantial length of a cased deviated borehole, it is necessary to align each of the perforating guns with adjacent perforating guns in the string to insure that the predetermined pattern of perforations is properly oriented within the deviated borehole. If the perforating guns are not all properly oriented with respect to each other, the perforation pattern will not be oriented in the proper direction within the cased borehole because one or more of the perforating guns is oriented in the wrong direction Although swivels can be used between adjacent perforating guns in a prior art perforating gun string, it is necessary that the swivels be watertight. Since the prima cord extends from the lowermost gun to the uppermost gun, the string of perforating guns must be hermetically sealed to prevent water from entering the gun string and damage the prima cord causing it to fail to detonate all of the guns. Further, it is necessary that packing be applied around the prima cord to keep it dry. In a long string of perforating guns, it is undesirable for the prima cord to bind or be sub]ect to pressure and friction from the packing of the prima cord connections between adjacent guns.
Other alignment systems for perforating guns include the rotation of the perforating gun within the cased borehole. Since the cased borehole is filled with well fluids such as mud, it is necessary that the perforating gun be sub]ected to this well fluid environment. Since the alignment system is subjected and exposed to well fluids, there is a possibility that the well fluids will hinder the proper alignment of the charges within the cased borehole.
The art of drilling a slanted borehole is being utilized to a much greater extent in recent years, as evidenced by U.S. Pat. No. 4,194,577, to which reference is made for further background of this invention.
In U.S. Pat. No. 4,194,577 and patent application Ser. No. 166,547 filed July 7, 1980, there is set forth a gravity oriented perforating tool. The gun has a radially extending guide means which contacts the inside peripheral wall of the casing and forces the gun into a substantially upright position. In some wells which are slanted at a substantial angle with respect to the horizontal, the gravitational forces are insufficient to overcome the frictional forces required to orient the before-mentioned perforating gun into a position whereby the shaped charges will fire downwardly in the desired direction for perforating the casing in a downward direction.
When the casing of a highly deviated borehole is perforated radially in all directions, some of the production occurs downwardly from the formation into the slanted portion of the borehole, thereby washing formation particles into the borehole, which is especially undesirable in unconsolidated formations. Therefore, it is desirable to have made available a low friction means by which the charge carriers of a perforating gun are journaled in low friction relationship with respect to the borehole so that reduced gravitational forces are required to orient the gun into a position whereby the shaped charges, when detonated, fire predominantly downwardly, thereby perforating the wellbore in a predominantly downward direction, whereupon production must then occur upwardly from the formation into the slanted part of the borehole.
Method and apparatus which achieves the above desirable goal is the subject of the present invention.